Low DP food casing from high solids viscose

ABSTRACT

A tubular food casing of a tubular cellulose film precipitated from a viscose solution having a viscosity of from about 55 to about 90 ball seconds, where the ball has a density of 8 g/cc and a radius of 0.316 centimeters at a drop of 20 centimeters, and where the solution contains at least eight and one-half weight percent of cellulose. The cellulose has a DPv of from about 300 to about 525 and the cellulose film has a dry film thickness of from about 0.015 mm to about 0.040 mm, a dry burst pressure in excess of 40 cm Hg, per 0.01 mm of dry film thickness, and a rewet burst pressure in excess of 5 cm Hg per 0.01 mm of rewet film thickness.

BACKGROUND OF THE INVENTION

The present invention relates to tubular food casings from cellulose films and more particularly relates to tubular food casings formed by extrusion of a solution of cellulose-followed by precipitation of the cellulose to form a tubular cellulose film.

In order to obtain a film that is strong enough and tough enough to be used as a food casing, e.g. for sausage casings, it has traditionally been believed that the cellulose had to have a relatively high molecular weight, e.g. as represented by its degree of polymerization (DP). The degree of polymerization that was believed to be required for a food casing of sufficient strength and toughness for commercial use was at least 560.

In the prior art, in order to dissolve cellulose, it was almost always first treated with sodium hydroxide to reduce the strength of hydrogen bonds and to expand it to permit the solvent to work more easily. Cellulose of sufficient DP to make a food casing, having good enough physical properties to be practical, still could not be dissolved to any significant degree in sodium hydroxide solution alone. However, there are no practical solvents for cellulose that function alone and such practical solvents, as do exist, usually require an alkali metal hydroxide as a cosolvent. Cellulose, for example, will not dissolve in aqueous carbon disulfide, or tertiary amine oxide, to any significant extent unless the cellulose is first expanded (steeped) in sodium hydroxide and the solution itself contains alkali metal hydroxide, preferably sodium hydroxide.

BRIEF DESCRIPTION OF THE INVENTION

The invention is a tubular food casing of a tubular cellulose film precipitated from a viscose solution having a viscosity of from about 55 to about 90 ball seconds, where the ball has a density of 8 g/cc and a radius of 0.316 cm, and where the solution contains at least eight and one-half weight percent of cellulose, said cellulose having a DPv of from about 300 to about 525. The cellulose film has a dry film thickness of from about 0.015 mm to about 0.040 mm, a dry burst pressure in excess of 40 cm Hg, per 0.01 mm of dry film thickness, and a rewet burst pressure in excess of 5 cm Hg per 0.01 mm of rewet film thickness.

The cellulose may be precipitated from solution of non-derivatized cellulose, e.g. from aqueous tertiary amine oxide solution or may be regenerated from a solution of derivatized cellulose, e.g. a solution of cellulose xanthate.

The invention also includes a method for making the cellulose film by:

a) preparing a viscose solution, containing at least eight and one-half weight percent of cellulose having a DPv of about 300 to about 525, and having a solution viscosity of from about 55 to about 90 ball seconds, where the ball has a density of 8 g/cc and a radius of 0.316 cm.

b) extruding the solution into the shape of a tube; and

c) precipitating cellulose from the extruded solution to form a tubular film having a dry film thickness of from about 0.015 mm to about 0.040 mm, a dry burst pressure in excess of 40 cm Hg, per 0.01 mm of dry film thickness, and a rewet burst pressure in excess of 5 cm Hg per 0.01 mm of rewet film thickness.

DETAILED DESCRIPTION OF THE INVENTION

The cellulose used in accordance with the invention has a low DPv, e.g. from about 300 to about 525 and usually from about 400 to about 475. The viscose (xanthate or traditional viscose) may be a derivatized cellulose, e.g. xanthanated with carbon disulfide, dissolved in caustic at a concentration of from about 4.5 to about 6.5 weight percent. The viscose total sulfur concentration is usually from about 1.8 to about 2.5 weight percent and to form a cellulose film, the cellulose is precipitated and regenerated from the xanthate by passing extruded viscose through a bath comprising a strong acid and a salt. The viscose may also be a solution comprising non-derivatized cellulose in a solvent comprising tertiary amine oxide and water (amine oxide viscose) obtained by forming a dilute solution of about 300 to about 525 DPv, preferably about 400 to about 475 DPv, cellulose and removing water by vaporization. The cellulose is precipitated by extruding the viscose and passing the extruded viscose through a wash bath containing water to remove tertiary amine oxide.

The viscose may also be a solution of non-derivatized cellulose in aqueous alkali. It has been surprisingly found that solutions of cellulose having low DPv can be obtained by dissolving specially prepared low DPv cellulose in dilute concentration in aqueous alkali followed by removing water, e.g. by vaporization under a partial vacuum, to obtain a cellulose solution in alkali (alkali viscose) having a high cellulose concentration, e.g. in excess of eight weight percent. In such a case, the viscose is obtained by forming a dilute solution of about 300 to about 525 DPv cellulose and removing the water by vaporization where the cellulose is obtained by treating higher DPv cellulose with acid or steam expansion to reduce the DPv.

Tubular cellulose film food casings made in accordance with the present invention have surprisingly good properties when compared with traditional cellulose film food casings made from high DPv cellulose. In particular such films may have a dry film thickness of from about 0.015 mm to about 0.040 mm, a dry burst pressure in excess of 40 cm Hg, per 0.01 mm of dry film thickness, and a rewet burst pressure in excess of 5 cm Hg per 0.01 mm of rewet film thickness.

Tubular food casings of the present invention may also include fiber reinforced films where the viscose is applied to a fiber web, e.g. a fiber paper or where fibers are blended into the viscose. Such tubular food casings are usually thicker and larger than unreinforced tubular film food casings.

The following examples serve to illustrate and not limit the present invention. Unless otherwise indicated, all parts and percentages are by total weight.

EXAMPLES 1-6

Cellulose having a degree of polymerization (DPv) of about 350 was dissolved at a concentration of about 9 percent in an aqueous solution of from about 5.3 to about 5.6 percent caustic and sufficient CS₂ to provide a xanthate sulfur value of from about 1.1 to about 1.5 percent by weight of cellulose with a total sulfur content of from about 1.95 to about 2 percent. The above cellulose solution (viscose) had a ball viscosity of from about 21 to about 39 seconds using a ball having a density of 8 g/cc, a radius of 0.316 cm and a drop of 20 cm. The unripened viscose had an adjusted maturity index of from about 10.2 to about 10.9. “Maturity index” is the number of ml of 10% acetic acid required to congeal the viscose. “Adjusted maturity index” is (viscose caustic wt. % −6.3)×0.3+measured maturity index. The unripened viscose (once filtered through a 10 micron filter) had a filterability K value of from about 2.54 to about 4.55 while viscose that had been ripened ( allowed to stand) and de-aerated under vacuum at 25° C. for more than about 24 hours and filtered twice had all adjusted ripened maturity of 7.9 and a K value of about 1.21. This was the ripened viscose used to form tubular film of the invention and is referred to in the examples as “low DP viscose”. “K value”=1000×[[(T₂−T₁)−(W₂−W₁)]/T₂−T1] where T₁ is the time of weighing of an 8 ounce sample (W₁) prior to filtering through a 4 ounce muslin filter cloth at a pressure of 60 psig. T₂ is the time of second weighing after filtration and W₂ is the weight of viscose at the second weighing.

The viscose at the high cellulose concentration of 8.9 to 9.2 percent surprisingly had a viscosity that was from about ⅓ to about ½ of the viscosity of standard viscose used to form tubular films. Such standard viscose is a solution of cellulose having a DPv of about 575 at a cellulose concentration of about 7.7 percent, a caustic concentration of about 6.3 percent, a xanthate sulfur concentration of about 1.15 and a total sulfur concentration of about 2.1. The standard viscose thus has a higher waste sulfur problem, a higher waste caustic problem, and a higher viscosity per percentage of dissolved cellulose than the viscose used in accordance with the invention, all of which result in processing advantages of using the low DPv viscose in accordance with the invention as opposed to standard high DPv viscose.

Viscose was extruded through a ring die having an internal ring diameter of about 25 mm and die gap of about 0.35 mm, referred to herein as a code 27 die, to form tubular cellulose film food casings. Both low DP and standard 575 DPv viscoses were used for purposes of comparison. Further various longitudinal stretches were used by varying uptake speed of extruded tubular film. Inflation with air at the pressure shown in Tables 1 and 2 was used to obtain transverse stretch. Viscose flow was adjusted so as to obtain a relatively uniform quantity of extruded cellulose for each of the food casings, i.e. flow for low DPv viscose through the die was about 956 grams (19.8 g/10 meters), while the flow for standard viscose, at lower solids, was about 813 grams per minute to obtain about the same quantity of cellulose solids in the film per unit area.

The extruded cellulose films were regenerated in baths containing a mixture of sodium sulfate and sulfuric acid. The concentrations were about 10.5% sulfuric acid and about 20% sodium sulfate. Less acid was consumed in the regeneration bath for low DP viscose than in the regeneration bath for standard viscose. The differences result because of higher solids concentration in the low DP viscose and lower sulfur and caustic loading in the low DP viscose.

Conditioned X-Y's means that the casing was conditioned at 80% relative humidity. “X-Y” refers to the plot of tube diameter against pressure. “RSD” means recommended stuffing diameter.

The results are shown in Tables 1-11.

EXAMPLE 7

A low DPv cellulose (about 350 DPv) was made by subjecting a high DPv cellulose (about 575 DPv) to a mineral acid. The acid was washed from the cellulose and the cellulose was dissolved in a caustic solution at a cellulose concentration of about 5 percent. Water is then removed from the cellulose solution under a vacuum to form a cellulose solution of about 8 percent. The resulting alkali viscose solution is then extruded to form a cellulose gel tubular film that is washed to remove alkali to form a tubular cellulose food casing.

EXAMPLE 8

Example 7 is repeated except that the DPv of the cellulose is reduced by enzymatic treatment with cellulase. An extrudable caustic solution of the resulting low DPv cellulose is then prepared as in Example 7 to prepare a tubular food casing.

EXAMPLE 9

Example 7 is repeated except that the DPv of the cellulose is reduced by treatment with concentrated-sodium hydroxide solution. The resulting low DPv cellulose does not dissolve in the caustic solution to an extent sufficient to permit formation of an extrudable viscose.

EXAMPLE 10

Example 7 is repeated except that the resulting viscose in extruded upon a cellulose fiber web rolled to form a tube to obtain a tubular fiber reinforced food casing.

The foregoing examples demonstrate that a low DPv cellulose can be used to make a practical tubular cellulose food casing without use of as much CS₂ as required in the known art and further that more readily available low DPv cellulose can be practically used. The invention further demonstrates that surprisingly CS₂ can be eliminated altogether when caustic is not used in prior treatment of cellulose to lower its DPv. This is entirely unexpected since traditional knowledge held that cellulose could not be dissolved in caustic alone in sufficient concentration to form an extrudable viscose. This misconception was due to the fact that cellulose was almost always treated with caustic prior to dissolution.

TABLE 1 Average Rewet X-Y's Code 27 Rewet Pressure Diam- Burst Residual Residual Flat Flat at RSD eter Pressure Burst Diameter Pressure Energy Thick- Exam- Sample Dryer Width Width (Cm at RSD (Cm Diameter Stretch Strength to Burst ness RSD ple Description Stretch Line (mm) (mm) Hg) (mm) Hg) (mm) (%) (%) (in lbs) (mm) (mm) 1 Low DPv −2.5% 140 35.0 34.0 15.0 23.7 29.1 33.0 34.8 94.1 6.0 0.072 24.5 Viscose-Not Filtered 2 Low DPv −2.5% 140 35.0 34.0 14.6 23.9 30.5 34.6 41.1 109.4 7.4 0.071 24.5 Viscose-10 u Filtered 3 Filtered Low 10.0% 140 35.0 34.0 12.6 24.2 30.4 34.2 39.5 141.6 6.9 0.066 24.5 DPv Viscose 4 Filtered Low −2.5% 140 35.0 34.0 14.0 24.0 30.7 32.8 34.0 118.4 6.2 0.068 24.5 DPv Viscose Std 5 Regular −2.5% 139 35.0 34.0 13.0 24.1 28.8 43.7 78.3 122.0 15.6 0.066 24.5 Production Viscose 6 Regular 10.0% 139 35.0 34.0 10.9 24.8 28.8 44.0 79.5 164.3 15.5 0.060 24.5 Production Viscose

TABLE 2 Average Conditioned X-Y's Code 27 Cond. Pressure Diam- Burst Residual Residual Flat Flat at RSD eter Pressure Burst Diameter Pressure Energy Thick- Exam- Sample Dryer Width Width (Cm at RSD (Cm Diameter Stretch Strength to Burst ness RSD ple Description Stretch Line (mm) (mm) Hg) (mm) Hg) (mm) (%) (%) (in lbs) (mm) (mm) 1 Low DPv −2.5% 140 35.0 35.0 65.4 77.4 27.1 14.0 26.3 7.5 0.033 24.5 Viscose-Not Filtered 2 Low DPv −2.5% 140 35.0 35.0 61.7 88.5 30.4 23.9 43.4 14.3 0.032 24.5 Viscose-10 u Filtered 3 Filtered Low 10.0% 140 35.0 35.0 54.2 86.1 30.1 22.7 59.2 13.2 0.032 24.5 DPv Viscose 4 Filtered Low −2.5% 140 35.0 35.0 65.5 102.2 31.5 28.6 55.9 19.1 0.034 24.5 DPv Viscose Std 5 Regular −2.5% 139 35.0 35.0 70.2 103.0 34.4 40.4 46.8 28.9 0.034 24.5 Production Viscose 6 Regular 10.0% 139 35.0 35.0 58.9 99.6 34.7 41.5 69.2 27.2 0.031 24.5 Production Viscose

TABLE 3 Average Rewet Longitudinal Instron Values Displace- Force to Elongation Energy Energy Force to ment Break Force @ Maxi- @ to at Break ½″ at user 1″ Modulus Maximum Maximum mum Maximum Break Break Exam- Sample Dryer Sample Break Sample @ 10% Modulus Tensile Tensile Tension Point 1″ ple Description Stretch Line (lbs) (in) (lbs) (psi) (psi) (lbs) (lbs) (%) (lbs-in) (lbs-in) 1 Low DPv −2.5% 140 3.9 0.9 7.8 11,890 12,410 3.9 3,016 28.4 1.9 3.7 Viscose-Not Filtered 2 Low DPv −2.5% 140 3.9 0.9 7.7  9,943 10,390 3.9 2,675 30.0 1.9 3.9 Viscose-10 u Filtered 3 Filtered Low 10.0% 140 5.0 0.9 9.9 20,250 20,050 5.0 3,969 27.0 2.3 4.5 DPv Viscose 4 Filtered Low −2.5% 140 4.8 1.1 9.6 11,710 11,730 4.8 3,549 35.4 2.8 5.6 DPv Viscose Std 5 Regular −2.5% 139 7.2 1.6 14.5 12,020 11,840 7.2 5,354 50.5 5.8 11.6 Production Viscose 6 Regular 10.0% 139 6.3 1.2 12.7 17,680 17,310 6.3 4,880 36.8 3.8 7.5 Production Viscose

TABLE 4 Average Rewet Transverse Instron Values Displace- Force to Elongation Force to ment Break 1″ Force @ @ Energy to Break 1″ at user Sample Modulus Maximum Maximum Maximum Maximum Break Sample Dryer Sample Break (gms/ @ 10% Modulus Tensile Tensile Tension Point Example Description Stretch Line (lbs) (in) 25 mm) (psi) (psi) (lbs) (lbs) (%) (lbs-in) 1 Low DPv −2.5% 140 6.1 0.9 5,517 3,081 5,760 6.1 2,346 64.9 2.5 Viscose-Not Filtered 2 Low DPv −2.5% 140 4.3 0.8 3,889 2,730 4,145 4.3 1,481 52.4 1.6 Viscose-10 u Filtered 3 Filtered Low 10.0% 140 6.2 1.0 5,640 2,386 6,194 6.2 2,493 68.5 2.5 DPv Viscose 4 Filtered Low −2.5% 140 6.7 1.0 6,120 3,162 5,968 6.8 2,506 65.0 2.8 DPv Viscose Std 5 Regular −2.5% 139 9.0 1.4 8,150 2,200 5,494 9.0 3,331 99.9 5.1 Production Viscose 6 Regular 10.0% 139 9.6 1.6 8,715 1,827 5,625 9.6 3,701 107.9 5.7 Production Viscose

TABLE 5 Average Conditioned Longitudinal Instron Values Displace- Force to Elongation Energy Energy Force to ment Break Force @ Maxi- @ to at Break ½″ at user 1″ Modulus Maximum Maximum mum Maximum Break Break Exam- Sample Dryer Sample Break Sample @ 10% Modulus Tensile Tensile Tension Point 1″ ple Description Stretch Line (lbs) (in) (lbs) (psi) (psi) (lbs) (lbs) (%) (lbs-in) (lbs-in) 1 Low DPv −2.5% 140 7.2 0.9 14.4 39,400  52,280 7.2 10,270 27.2 3.9 7.0 Viscose-Not Filtered 2 Low DPv −2.5% 140 6.9 0.9 13.8 39,470  57,010 6.9 10,590 28.4 4.0 8.0 Viscose-10 u Filtered 3 Filtered Low 10.0% 140 8.8 0.7 17.6 69,380 119,800 8.8 15,970 20.9 3.9 7.7 DPv Viscose 4 Filtered Low −2.5% 140 8.3 0.9 16.6 48,640  73,040 8.3 13,840 29.4 5.0 9.9 DPv Viscose Std 5 Regular −2.5% 139 10.8 1.3 21.6 56,210  66,850 10.8 19,620 40.2 8.8 17.5 Production Viscose 6 Regular 10.0% 139 10.0 0.7 20.0 75,880 119,700 10.0 18,210 23.4 4.8 9.6 Production Viscose

TABLE 6 Average Conditioned Transverse Instron Values Force to Elongation Force to Displace- Break 1″ Force @ @ Energy to Break 1″ ment Sample Modulus Maximum Maximum Maximum Maximum Break Sample Dryer Sample at user (gms/ @ 10% Modulus Tensile Tensile Tension Point Example Description Stretch Line (lbs) Break 25 mm) (psi) (psi) (lbs) (lbs) (%) (lbs-in) 1 Low DPv −2.5% 140 8.0 0.5 7,282 15,410 109,200 8.0 5,735 35.2 3.0 Viscose-Not Filtered 2 Low DPv −2.5% 140 7.7 0.5 6,971 17,200  82,130 7.7 5,912 34.5 2.8 Viscose-10 u Filtered 3 Filtered Low 10.0% 140 9.6 0.8 8,686 15,580  44,120 9.6 8,708 56.1 5.1 DPv Viscose 4 Filtered Low −2.5% 140 11.0 0.7 10,010  20,970 111,800 11.0 9,194 45.9 5.2 DPv Viscose Std 5 Regular −2.5% 139 7.9 0.4 7,184 18,510 232,400 7.9 7,199 25.4 2.1 Production Viscose 6 Regular 10.0% 139 6.5 0.5 5,879 13,030 157,600 6.5 5,892 31.5 2.1 Production Viscose

TABLE 7 Example Sample Description Dryer Stretch Line Birefringence BDG DPv Gel Check Glycerine Total Sulfur (ppm) 1 Low Dpv Viscose-Not Filtered −2.5% 140 0.0077 21.6 310 No Gels 25.50 1502 2 Low DPv Viscose-10 u Filtered −2.5% 140 0.0072 21.4 396 No Gels 26.20 1434 3 Filtered Low DPv Viscose 10.0% 140 0.0115 20.0 385 No Gels 26.28 1472 4 Filtered Low DPv Viscose Std −2.5% 140 0.0071 21.8 308 No Gels 24.81 1527 5 Regular Production Viscose −2.5% 139 0.0092 22.1 577 No Gels 20.37 1748 6 Regular Production Viscose 10.0% 139 0.0139 20.6 575 No Gels 20.81 1740 Standard or Typical Values 21.7 585

TABLE 8 Example Sample Description Dryer Stretch Line Permeability pH Avg. % Skin Average Outside Skin Average Inside Skin 1 Low Dpv Viscose-Not Filtered −2.5% 140 416 8.8 4.96 <250 nm 1.39 2 Low DPv Viscose-10 u Filtered −2.5% 140 450 8.6 11.58 1.68 1.21 3 Filtered Low DPv Viscose 10.0% 140 440 8.8 7.59 0.95 1.10 4 Filtered Low DPv Viscose Std −2.5% 140 273 8.7 13.42 2.34 1.28 5 Regular Production Viscose −2.5% 139 245 8.6 14.15 2.35 1.33 6 Regular Production Viscose 10.0% 139 233 9.0 9.14 1.31 1.16 Standard or Typical Values 270

TABLE 9 Peeling Comments Test Emulsion Water Starch Casing Stuffing Stuffing Stuffmg Slip/ Peeling Peeled immediately ID Type Content Type Description Diameter Temperature Comments No Slip Performance after cooking 5 Chicken 25% 12.5% Standard Viscose/ 25.5 mm 58° F. 0 defects Good 100% 0 Internal temperature after tap (159#) Standard Stretch misses out of water shower of 111° F. and 84 hot dogs out of 7 min. bath at 54° F. 2 Low DPv 25.5 mm 1 linker Good 100% 0 misses out of 81 hot dogs Viscose/ break Standard Stretch 6 Standard Viscose/ 25.8 mm 0 defects Good 100% 0 misses out of 80 hot dogs. 10% Stretch 3 Low DPv Vis- 25.9 mm 1 linker Good 100% 0 misses out of 67 hot dogs. cose/10% Stretch break

TABLE 10 Peeling Comments Test Emulsion Water Starch Casing Stuffing Stuffing Stuffmg Slip/ Peeling after cooking hold 45 minutes ID Type Content Type Description Diameter Temperature Comments No Slip Performance at 70° F. prior to peeling 5 Chicken 25% 12.5% Standard Viscose/ 25.5 mm 58° F. 0 defects None 21% 74 Staging time at 45 minutes. (159#) Standard Stretch misses out of Internal temperature after 94 hot dogs staging at 77° F. and out of 7 min. bath at 47° F. 2 Low DPv 25.5 mm 0 defects None  8% 83 misses out of 90 hot dogs. Viscose/ Standard Stretch 6 Standard Viscose/ 25.8 mm 0 defects None 24% 71 misses out of 94 hot dogs. 10% Stretch 3 Low DPv Vis- 25.9 mm 0 defects None 18% 73 misses out of 89 hot dogs. cose/10% Stretch

TABLE 11 Peeling Comments Test Emulsion Water Starch Casing Stuffing Stuffing Stuffmg Slip/ Peeling after cooking hold 90 minutes ID Type Content Type Description Diameter Temperature Comments No Slip Performance at 70° F. prior to peeling 5 Chicken 25% 12.5% Standard Viscose/ 25.6 mm 58° F. 0 defects None 4% 82 Staging time at 90 minutes. (159#) Standard Stretch misses out of Internal temperature after 85 hot dogs staging at 78° F. and out of 7 min. bath at 43° F. 2 Low DPv 25.5 mm 0 defects None 6% 82 misses out of 87 hot dogs. Viscose/ Standard Stretch 6 Standard Viscose/ 25.7 mm 0 defects None 1% 85 misses out of 86 hot dogs. 10% Stretch 3 Low DPv Vis- 25.9 mm 1 split out None 0% 74 misses out of 74 hot dogs. cose/10% Stretch Comments: *Evaluate casing produced on PM 10½ with standard viscose and with (high viscose cellulose and low DPv) Low DPv viscose in standard dryer stretch & T400 mode. *100% Chicken, 25% water (based on chicken weight); 12.5% corn starch (based on chicken weight), with Heller Seasonings (9.6 lbs). *Cook cycle: 150 F. DB/0 F. WB for 15 min; 158 F. DB/158 F. WB for 30 min; 167 F. DB/167 F. WB for 30 min; 172 F. DB/172 F. WB for 15 min; 176 F. DB/176 F. WB for 7 min; 176 F. DB/0 F. WB for 1 min; tap water shower with door open 6″ until 111 F. int. temp is reached; vary staging time 0, 45 & 90 min; then 7 min chilled bath. 

What is claimed is:
 1. A tubular food casing comprising a tubular cellulose film precipitated from a viscose solution having a viscosity of from about 55 to about 90 ball seconds, where the ball has a density of 8 g/cc and a radius of 0.316 centimeters at a drop of 20 centimeters, and where the solution contains at least eight and one-half weight percent of cellulose, said cellulose having a DPv of from about 300 to about 525, said cellulose film having a dry film thickness of from about 0.015 mm to about 0.050 mm, a dry burst pressure in excess of 40 cm Hg, per 0.01 mm of dry film thickness, and a rewet burst pressure in excess of 5 cm Hg per 0.0 mm of rewet film thickness.
 2. The food casing of claim 1 wherein the cellulose has a DPv of about 400 to about 475 and the dry film thickness is from about 0.015 mm to about 0.040 mm.
 3. The food casing of claim 1 wherein the viscose is a xanthate viscose containing a caustic concentration of from about 4.5 to about 6.5 weight percent and a viscose total sulfur concentration of from about 1.8 to about 2.5 weight percent and the cellulose is precipitated by passing extruded viscose through a bath comprising a strong acid and a salt.
 4. The food casing of claim 1 wherein the viscose is a solution comprising non-derivatized cellulose in a solvent comprising tertiary amine oxide and water obtained by forming a dilute solution of about 300 to about 525 DPv cellulose and removing water by vaporization and the cellulose is precipitated by passing extruded viscose through a wash bath comprising water to remove tertiary amine oxide.
 5. The food casing of claim 4 wherein the water is removed in a partial vacuum.
 6. The food casing of claim 1 wherein the viscose comprises a non-derivatized cellulose in a solvent comprising water and sodium hydroxide obtained by forming a dilute solution of about 300 to about 525 DPv cellulose and removing the water by vaporization where the cellulose is obtained by treating higher DPv cellulose with acid to reduce the DPv.
 7. The food casing of claim 6 wherein the water is removed in a partial vacuum.
 8. The food casing of claim 6 wherein the cellulose is precipitated by passing extruded viscose through a wash bath comprising water to remove sodium hydroxide.
 9. A method for making the tubular food casing of claim 1 which comprises: a) preparing a viscose solution, containing at least eight and one-half weight percent of cellulose having a DPv of about 300 to about 525, and having a solution viscosity of from about 55 to about 90 ball seconds, where the ball has a density of 8 grams per cubic centimeter and a radius of 0.316 cm, at a drop of 20 centimeter; b) extruding the solution into the shape of a tube; and c) precipitating cellulose from the extruded solution to form a tubular film having a dry film thickness of from about 0.015 mm to about 0.050 mm, a dry burst pressure in excess of 40 cm Hg, per 0.01 mm of dry film thickness, and a rewet burst pressure in excess of 5 cm Hg per 0.0 1 mm of rewet film thickness.
 10. The method of claim 9 wherein the cellulose has a DPv of about 425 to less than
 500. 11. The method of claim 9 wherein the viscose is a xanthate viscose containing a caustic concentration of from about 4.5 to about 6.5 weight percent and a viscose total sulfur concentration of from about 1.8 to about 2.5 weight percent and the cellulose is precipitated by passing extruded viscose through an aqueous bath comprising acid and a salt.
 12. The method of claim 9 wherein the viscose is a solution comprising non-derivatized cellulose in a solvent comprising tertiary amine oxide and water obtained by forming a dilute solution of about 300 to about 525 DPv cellulose and removing water by vaporization and the cellulose is precipitated by passing extruded viscose through a wash bath comprising water to remove tertiary amine oxide.
 13. The method of claim 12 wherein the water is removed in a partial vacuum.
 14. The method of claim 9 wherein the viscose comprises a non-derivatized cellulose in a solvent comprising water and sodium hydroxide obtained by forming a dilute solution of about 300 to about 525 DPv cellulose and removing the water by vaporization.
 15. The method of claim 14 wherein the water is removed in a partial vacuum.
 16. The method of claim 15 wherein the cellulose is precipitated by passing extruded viscose through a wash bath comprising water to remove sodium hydroxide.
 17. The method of claim 9 wherein the dissolving cellulose has a DPv of between about 300 to about
 400. 18. The method of claim 9 where the dry film thickness is from about 0.015 mm to about 0.040 mm.
 19. The method of claim 9 wherein the viscose is an alkali solution of cellulose formed from a higher DPv cellulose by steam.
 20. The method of claim 9 wherein the viscose is an alkali solution of cellulose formed from a higher DPv cellulose by treatment with acid. 